It happens to almost every boy and girl somewhere between junior high and the early years of high school. You have a teacher that is quite young, and almost always "cool." This is usually their first teaching job out of college. They are finding their way around teaching as much as you were finding your way around the raging sea of hormones that accompanies puberty. These young teachers are adults, but they aren't THAT much older than you. Depending on which grade you are in, the difference might be only seven or eight years. You girls can think of several couples where the age difference is that much or more.
For the boys, it is the visualization of them with a woman with fully developed sexual organs. Although it has been established that the often quoted fact of men thinking about sex every seven seconds has been proved to be urban legend, I think the number of times adolescent males think of it is much higher than older males. For the girls this time is often the first chance they have to try out their flirting skills. Most would only do that with a single young teacher, because the family embedded taboo of being a home wrecker is too evil to cross. Most guys aren't that concerned whether the female teacher is married or not.
This story isn't about my particular young teacher I had a crush on. There were probably several, but I don't' even remember their names or faces anymore. The story is about a woman that I was engaged to. She was a teacher in junior high and had gotten her degree in sociology. She came home in a rather fetching if not rather short dress and I made a comment about the boys must have been dropping pencils a lot today.
The look on her face was total confusion. She had no idea of what I was talking about. My reaction was total disbelief. She admitted an unusual number of boys had dropped their pencils, papers, or books today, but she didn't think anything about it. When I finally stopped laughing, what was obviously much too long a time later, I was confronted with a fiancée demanding answers. I explained the male art of dropping things from their desk, so they could look up girls dresses. There was the drop and tuck, for the girls sitting behind you and the drop and roll for those seated in front of you. This was especially good if the girl was sitting opposite of you.
She still wasn't sure if I was kidding or not, because I have been known to be more than a little bit of a practical joker. So we were off to the computer to Google the subject. Needless to say there were numerous examples of guys blogging about it, and teacher's references to it. I assume that 99.9 percent of guys know about it, and the majority of those have done it as well. Her naiveté was written off to being educated in an all girls Catholic school. How she missed it in sociology studies escapes me. But the kicker was that when we Googled "dropping pencils", there were as many references to girls dropping pencils to see if the teacher picked them up or dropping pencils to provide their teacher a view down their shirts, as there were of boys dropping pencils.